The present invention relates generally to multiple frequency communication systems, and more particularly to a multiple frequency communication system that utilizes predetermined message formats and codewords to transmit information. Typical multiple frequency communication systems include frequency hopping communication systems, and spread spectrum communication systems, as well as digital communication systems that transmit simultaneously on many frequencies.
An example of a frequency hopping communication system is the U.S. Army's SINCGARS communication system, which is a frequency hopping communication system that utilizes predetermined message formats and information fields. This frequency hopping communication system, as well as all frequency hopping communication systems, are susceptible to interference from nearby communication systems transmitting at frequencies being used by the frequency hopping communication system. Typically, this type of interference causes data errors in the received bit sequence.
One method of preventing interference relies on identifying frequencies of potential interference sources and avoiding those frequencies during transmission. This method can only identify interfering sources that have previously occurred, and not those that occur only during a particular communication session.
Furthermore, this method requires performance of an RF survey of the area in which the communication session is to occur. This RF survey may occur long before a particular communication session occurs. One result is that if signals that were identified as potential interferers are no longer transmitting during a particular communication session, one has unnecessarily limited the choice of frequencies during that particular communication session. Another result is that frequencies that were identified as interference free frequencies may later contain interference sources that cause interference during a particular communication session. Furthermore, RF surveys are inherently fraught with error due to today's dynamic RF environment, realistic time and resource limitations, site access limitations and changing transmission bandwidths of new communication systems. In an attempt to take into account the dynamic RF environment, there have been attempts to use data from old RF surveys to project what the potential interference sources might be for a transmission link occurring in a particular location, even if the RF survey was not intended to gather data applicable to that location, or frequency bandwidth. The accuracy of such interpolation is dubious, at best. In any event, any RF survey presupposes one knows the locations in which the communication system will be used. However, a designer may not know all possible locations in which the communication system may be used. In fact, a communication system must be versatile and must allow its user the freedom to move between locations. Therefore the use of RF surveys has its limitations.
Finally, even an RF survey cannot determine the actual RF pattern for all transmitting and receiving locations and for all frequencies of interest. RF transmission patterns contain nulls at certain frequencies and location that prevent successful transmission. These nulls can arise from multipath transmission, for example, and are a function of many variables, such as the exact geography of the transmitting and receiving locations. This phenomena is often experienced by car phone users, whose received signal seems to fade in and out as the car travels. Therefore, although all frequencies containing potential interference sources may have been successfully identified and blocked out using an RF survey, reliable transmission may still be impossible at particular locations, which are unknown prior to attempting transmission.
The present invention is directed to the problem of developing a multiple frequency communication system and a method of operating a multiple frequency communication system that allows a user to transmit on all available frequencies, yet avoids those frequencies that will cause data errors in the transmission, on a dynamic basis. The present invention is also directed to the problem of developing a multiple frequency communication system that does not rely upon RF surveys to determine the frequencies of potential interference sources, and thus operates independently of the local RF environment. Finally, the present invention is directed to the problem of improving the data reliability of a frequency hopping communication system without increasing the complexity of the transmitter.